1. Field of the Invention
The present invention relates to data transmission systems, and more particularly, to data transmission systems utilizing time-division duplexing.
2. Description of the Related Art
Bi-directional digital data transmission systems are presently being developed for high-speed data communications. One standard for high-speed data communications over twisted-pair phone lines that has developed is known as Asymmetric Digital Subscriber Lines (ADSL). Another standard for high-speed data communications over twisted-pair phone lines that is presently proposed is known as Very High Speed Digital Subscriber Lines (VDSL). In general, these high-speed data communications techniques are referred to as xDSL systems.
The Alliance For Telecommunications Information Solutions (ATIS), which is a group accredited by the ANSI (American National Standard Institute) Standard Group, has finalized a discrete multi tone based approach for the transmission of digital data over twisted-pair phone lines. The standard, known as ADSL, is intended primarily for transmitting video data and fast Internet access over ordinary telephone lines, although it may be used in a variety of other applications as well. The North American Standard is referred to as the ANSI T1.413 ADSL Standard (hereinafter ADSL standard), and is hereby incorporated by reference. Transmission rates under the ADSL standard are intended to facilitate the transmission of information at rates of up to 8 million bits per second (Mbits/s) over twisted-pair phone lines. The standardized system defines the use of a discrete multi tone (DMT) system that uses 256 xe2x80x9ctonesxe2x80x9d or xe2x80x9csub-channelsxe2x80x9d that are each 4.3125 kHz wide in the forward (downstream) direction. In the context of a phone system, the downstream direction is defined as transmissions from the central office (typically owned by the telephone company) to a remote location that may be an end-user (i.e., a residence or business user). In other systems, the number of tones used may be widely varied.
The ADSL standard also defines the use of reverse transmissions at a data rate in the range of 16 to 800 Kbit/s. The reverse transmissions follow an upstream direction, as for example, from the remote location to the central office. Thus, the term ADSL comes from the fact that the data transmission rate is substantially higher in the downstream direction than in the upstream direction. This is particularly useful in systems that are intended to transmit video programming or video conferencing information to a remote location over telephone lines.
Because both downstream and upstream signals travel on the same pair of wires (that is, they are duplexed) they must be separated from each other in some way. The method of duplexing used in the ADSL standard is Frequency Division Duplexing (FDD) or echo canceling. In frequency division duplexed systems, the upstream and downstream signals occupy different frequency bands and are separated at the transmitters and receivers by filters. In echo cancel systems, the upstream and downstream signals occupy the same frequency bands and are separated by signal processing.
ANSI is producing another standard for subscriber line based transmission system, which is referred to as the VDSL standard. The VDSL standard is intended to facilitate transmission rates of at least about 4 Mbit/s and up to about 52 Mbit/s or greater in the downstream direction. Simultaneously, the Digital, Audio and Video Council (DAVIC) is working on a similar system, which is referred to as Fiber To The Curb (FTTC). The transmission medium from the xe2x80x9ccurbxe2x80x9d to the customer is standard unshielded twisted-pair (UTP) telephone lines.
A number of modulation schemes have been proposed for use in the VDSL and FTTC standards (hereinafter VDSL/FTTC). For example, some of the possible VDSL/FTTC modulation schemes include multi-carrier transmission schemes such as Discrete Multi-Tone modulation (DMT) or Discrete Wavelet Multi-Tone modulation (DWMT), as well as single carrier transmission schemes such as Quadrature Amplitude Modulation (QAM), Carrierless Amplitude and Phase modulation (CAP), Quadrature Phase Shift Keying (QPSK), or vestigial sideband modulation.
Additionally, multicarrier modulation transmission schemes have been receiving a large amount of attention due to the high data transmission rates they offer. FIG. 1A is a simplified block diagram of a conventional transmitter 100 for a multicarrier modulation system. The conventional transmitter 100 is, for example, suitable for DMT modulation in ADSL or VDSL systems. The transmitter 100 receives data signals to be transmitted at a buffer 102. The data signals are then supplied from the buffer 102 to a forward error correction (FEC) unit 104. The FEC unit 104 compensates for errors that are due to crosstalk noise, impulse noise, channel distortion, etc. The signals output by the FEC unit 104 are supplied to a data symbol encoder 106. The data symbol encoder 106 operates to encode the signals for a plurality of frequency tones associated with the multicarrier modulation. In assigning the data, or bits of the data, to each of the frequency tones, the data symbol encoder 106 utilizes data stored in a transmit bit allocation table 108 and a transmit energy allocation table 110. The transmit bit allocation table 108 includes an integer value for each of the carriers (frequency tones) of the multicarrier modulation. The integer value indicates the number of bits that are to be allocated to the particular frequency tone. The value stored in the transmit energy allocation table 110 is used to effectively provide fractional number of bits of resolution via different allocation of energy levels to the frequency tones of the multicarrier modulation. In any case, after the data symbol encoder 106 has encoded the data onto each of the frequency tones, an Inverse Fast Fourier Transform (IFFT) unit 112 modulates the frequency domain data supplied by the data symbol encoder 106 and produces time domain signals to be transmitted. The time domain signals are then supplied to a digital-to-analog converter (DAC) 114 where the digital signals are converted to analog signals. Thereafter, the analog signals are transmitted over a channel to one or more remote receivers.
FIG. 1B is a simplified block diagram of a conventional remote receiver 150 for a multicarrier modulation system. The conventional remote receiver 150 is, for example, suitable for DMT demodulation in ADSL or VDSL systems. The remote receiver 150 receives analog signals that have been transmitted over a channel by a transmitter. The received analog signals are supplied to an analog-to-digital converter (ADC) 152. The ADC 152 converts the received analog signals to digital signals. The digital signals are then supplied to a Fast Fourier Transform (FFT) unit 154 that demodulates the digital signals while converting the digital signals from a time domain to a frequency domain. The demodulated digital signals are then supplied to a frequency domain equalizer (FEQ) unit 156. The FEQ unit 156 performs an equalization on the digital signals so the attenuation and phase are equalized over the various frequency tones. Then, a data symbol decoder 158 receives the equalized digital signals. The data symbol decoder 158 operates to decode the equalized digital signals to recover the data, or bits of data, transmitted on each of the carriers (frequency tones). In decoding the equalized digital signals, the data symbol decoder 158 needs access to the bit allocation information and the energy allocation information that were used to transmit the data. Hence, the data symbol decoder 158 is coupled to a received bit allocation table 162 and a received energy allocation table 160 which respectively store the bit allocation information and the energy allocation information that were used to transmit the data. The data obtained from each of the frequency tones is then forwarded to the forward error correction (FEC) unit 164. The FEC unit 164 performs error correction of the data to produce corrected data. The corrected data is then stored in a buffer 166. Thereafter, the data may be retrieved from the buffer 166 and further processed by the receiver 150. Alternatively, the received energy allocation table 160 could be supplied to and utilized by the FEQ unit 166.
The bit allocation tables and the energy allocation tables utilized in the conventional transmitter 100 can be implemented as a single table or as individual tables. Likewise, the bit allocation tables and the energy allocation tables utilized in the remote receiver 150 can be implemented as a single table or as individual tables. Also, the transmitter 100 is normally controlled by a controller, and the remote receiver 150 is normally controlled by a controller. Typically, the controllers are programmable controllers.
The transmitter 100 and the remote receiver 150 illustrated in FIGS. 1A and 1B, respectively, optionally include other components. For example, the transmitter 100 could add a cyclic prefix to symbols after the IFFT unit 112, and the remote receiver 150 can then remove the cyclic prefix before the FFT unit 154. Also, the remote receiver 150 can provide a time domain equalizer (TEQ) unit between the ADC 152 and the FFT unit 154. The transmitter 100 could also each respectively include a scrambler, a cyclic redundancy check (CRC) and an interleaver, with the remote receiver 150 including counterpart devices.
Most of the proposed VDSL/FTTC transmission schemes utilize frequency division duplexing (FDD) of the upstream and downstream signals. On the other hand, one particular proposed VDSL/FTTC transmission scheme uses time division duplexing (TDD) of the upstream and downstream signals. More particularly, the time division duplexing is synchronized in this case such that periodic synchronized upstream and downstream communication periods do not overlap with one another. That is, the upstream and downstream communication periods for all of the wires that share a binder are synchronized. With this arrangement, all the very high speed transmissions within the same binder are synchronized and time division duplexed such that downstream communications are not transmitted at times that overlap with the transmission of upstream communications. This is also referred to as a (i.e. xe2x80x9cping pongxe2x80x9d) based data transmission scheme. Quiet periods, during which no data is transmitted in either direction, separate the upstream and downstream communication periods. When the synchronized time division duplexed approach is used with DMT it is often referred to as synchronized DMT (SDMT).
A common feature of the above-mentioned transmission systems is that twisted-pair phone lines are used as at least a part of the transmission medium that connects a central office (e.g., telephone company) to users (e.g., residence or business). Even though fiber optics may be available from a central office to the curb near a user""s residence, twisted-pair phone lines are used to bring in the signals from the curb into the user""s home or business.
The twisted-pair phone lines are grouped in a binder. While the twisted-pair phone lines are within the binder, the binder provides reasonably good protection against external electromagnetic interference. However, within the binder, the twisted-pair phone lines induce electromagnetic interference on each other. This type of electromagnetic interference is generally known as crosstalk interference which includes near-end crosstalk (NEXT) interference and far-end crosstalk (FEXT) interference. As the frequency of transmission increases, the crosstalk interference (NEXT interference) becomes substantial. As a result, the data signals being transmitted over the twisted-pair phone lines at high speeds can be significantly degraded by the crosstalk interference caused by other twisted-pair phone lines in the binder. As the speed of the data transmission increases, the problem worsens. One advantage of the synchronized TDD (such as SDMT) based data transmission is that crosstalk interference (NEXT interference) from other lines in a binder is essentially eliminated, provided all the lines transmit for the same duration (i.e., same superframe format).
A data transmission system often includes a central office and a plurality of remote units. Each remote unit communicates with the central office over a data link (i.e., channel) that is established between the central office and the particular remote unit. To establish such a data link, initialization processing is performed to initialize communications between the central office and each of the remote units. For purposes of the discussion to follow, a central office includes a central modem (or central unit) and a remote unit includes a remote modem. These modems are transceivers that facilitate data transmission between the central office and the remote unit. The central office thus often includes a plurality of central side transceivers, each of which has a central side transmitter and a central side receiver, and the remote unit normally includes a remote side transceiver having a remote side transmitter and a remote side receiver.
When a data transmission system is operating in a time division duplexed (TDD) manner, the transmitters and receivers of the central office and remote units must be synchronized in time so that transmission and reception do not overlap in time. In a data transmission system, downstream transmissions are from a central side transmitter to one or more remote side receivers and upstream transmissions are from one or more remote side transmitters to a central side receiver. The central side transmitter and receiver can be combined as a central side transceiver, and the remote side transmitter and receiver can be combined as a remote side transceiver.
Generally speaking, in a time division duplexed system, upstream signals are alternated with downstream signals. Typically, the upstream transmissions and the downstream transmissions are separated by a guard interval or a quiet period. The guard interval is provided to enable the transmission system to reverse the direction in which data is being transmitted so that a transmission can be received before the transmission in the opposite direction occurs. Some transmission schemes divide upstream and downstream transmissions into smaller units referred to as frames. These frames may also be grouped into superframes that include a series of downstream frames and a series of upstream frames, as well as guard intervals between the two.
Time division duplexing is a simple method to share a channel (medium) between two or more transceivers. Each transceiver is assigned a time slot during which it may transmit, and there are quiet periods (guard intervals) during which no unit must transmit. On channels subject to crosstalk (NEXT interference) between multiple connections, if time division duplexing is used, synchronization must be established and maintained among all units so affected. An example is the VDSL service that uses the existing twisted pair telephone loop plant to transport up to 13-52 Mb/s on loops up to 1.5 km. Pairs destined for subscribers are bundled together in a cable consisting of 25-100 pairs. The proximity and the high frequency use (0.2-11 MHz signal bandwidth) leads to significant crosstalk between adjacent pairs in a binder. To get the desired data rate on loops up to several km long, DMT is a suitable multicarrier modulation scheme. This scheme makes excellent use of time-division duplexing since a single FFT unit can be used during transmission and reception and avoids the need for two such FFT units, and other savings in the analog circuitry.
FIG. 2A is a diagram of a superframe 200 suitable for use in VDSL data transmission systems. SDMT is a multicarrier data transmission scheme in which each of a plurality of frames included in the superframe 200 utilize a plurality of frequency tones to carry bits of data between a transmitter and receiver. More particularly, the superframe 200 includes twenty (20) frames. Frames 1-9 are used to transmit data in a downstream direction, frames 11-19 are used to transmit data in an upstream direction, and frames 10 and 20 are quiet periods. During the quiet periods data is not transmitted in either direction. Accordingly, as seen by the superframe 200, the VDSL data transmission systems have alternating periods of transmission and reception which operate to provide time division duplexing (TDD) transmission.
Although FIG. 2A illustrated a superframe as having twenty (20) frames, it should be recognized that a wide variety of superframe sizes and formats are available. By altering the number of the frames (or symbols) assigned to upstream and downstream directions, different formats for a superframe can be obtained and these different formats result in different levels of service being provided to the transmitter and receiver.
Additional details on data communications can be found, for example, in U.S. Pat. Nos. 5,479,447; 5,596,604; 5,623,513; and 5,627,863, which are hereby incorporated by reference. Additional details on ADSL can be found in American Nationals Standard Institution (ANSI) published standard ANSI T1.413-1995 pertaining to Network and Customer Installation Interfacesxe2x80x94Asymmetric Digital Subscriber Lines (ADSL) Metallic Interface, which is hereby incorporated by reference.
One problem with TDD systems such as SDMT is that the number of frames per superframe that transmit in any given direction is variable, and thus such systems do not have any constant average symbol rate. As a consequence, setting the user data rate tends to be more complicated and has more irregular granularity than conventionally provided in existing lower speed communication methods such as SDMT data communication systems. In ADSL and previously proposed VDSL data communication systems, there is a constant symbol rate which in turn translates to the fact that a user can choose any data rate in multiples of 32 k bits/seconds (kbps). Accordingly, there is a need for improvements with VDSL data communication systems so that the user data rate is granular (i.e., as granular as ADSL) but does not depend on internal system issues like the size of the superframe or the number of downstream frames in a superframe.
Another problem associated with TDD and SDMT data communication systems is that undesirable delays can occur if codewords (resulting from error correction processing) for a particular superframe end up crossing a superframe boundary during transmission. In such a case, there is an undesired time delay associated with the reception of a particular codeword that crosses a superframe boundary because of the predetermined time gap until the next transmission period due to the alternating transmissions and receptions of data in a TDD system. Such a problem is not present in ADSL since ADSL relies on frequency division duplexing (FDD) or echo canceling to provide the separation between upstream and downstream transmissions (as opposed to time division duplexing (TDD)).
Yet another problem associated with VDSL data communication systems is that the coding gain of error correction (i.e., FEC) is strongly dependent on the size of the codewords. Generally speaking, the larger the codeword size (N), the greater the coding gain. For example, a Reed Solomon code with N=255 and K=239 out performs a similar code with N=155 and K=139 (with Nxe2x88x92K=16 in both cases) by 3 dB under typical conditions. In ADSL the frames or symbols are fixed with respect to a codeword at 1, 2, 4, 8 or 16 symbols per codeword. However, if in VDSL the codeword size or number of symbols per symbol were fixed, the coding gain would ends up being dependent on the superframe rate, symbols per superframe or other internal system issues which is not desirable.
Thus, there is a need for improved approaches to provide flexibility in setting user data rates and managing delay in data transmission systems using a superframe structure and Time Division Duplexing (TDD).
Broadly speaking, the invention relates to improved approaches to provide flexibility in setting user data rates and managing delay in data transmission systems using a superframe structure and Time Division Duplexing (TDD). These improved approaches operate to provide intelligent insertion of dummy words (bits or bytes) into a data stream to be transmitted. By inserting the dummy words, the invention is able to render codewords, symbols and superframes independent from user data rates. As a result, a wide range of user data rates are available in data transmission systems using a superframe and TDD.
The invention can be implemented in numerous ways, including as an apparatus, system, method, or computer readable media. Several embodiments of the invention are discussed below.
As a method for transmitting a data quantity in a superfame of a multicarrier modulation system, one embodiment of the invention includes the operations of: identifying a user rate for data transmissions; identifying a superframe rate; determining a data quantity to be transmitted in a given superframe based on the user rate and the superframe rate; determining a code rate for providing redundancy with the data quantity; and determining a first adjustment dummy data quantity such that when added to the data quantity and then multiplied by the code rate yields an integer number.
As a method for transmitting a data quantity in a superframe of a multicarrier modulation system, where the superframe including transmit frames and receive frames, another embodiment of the invention includes the operations of: identifying a user rate for data transmissions; identifying a superframe rate; determining a data quantity to be transmitted in the superframe based on the user rate and the superframe rate; determining a code rate (e.g., via codeword size and dataword size) for providing redundancy with the data quantity; determining a first adjustment dummy data quantity such that when added to the data quantity and then multiplied by the code rate yields an integer number representing an enlarged data quantity to be transmitted in the superframe; and determining a second adjustment dummy data quantity such that when added to the enlarged data quantity to produce a final data quantity, the final data quantity is evenly divisible by the number of transmit frames in the superframe.
As a computer readable medium containing program instructions for transmitting a data quantity in a superfame of a multicarrier modulation system, the superframe including transmit frames and receive frames, one embodiment of the invention includes: first computer readable code devices for identifying a user rate for data transmissions; second computer readable code devices for identifying a superframe rate; third computer readable code devices for determining a data quantity to be transmitted in the superframe based on the user rate and the superframe rate; fourth computer readable code devices for determining a code rate for providing redundancy with the data quantity; and fifth computer readable code devices for determining a first adjustment dummy data quantity such that when added to the data quantity and then multiplied by the code rate yields an integer number representing an enlarged data quantity to be transmitted in the superframe. Optionally, the computer readable medium can also include sixth computer readable code devices for determining a second adjustment dummy data quantity such that when added to the enlarged data quantity to produce a final data quantity, the final data quantity is evenly divisible by the number of transmit frames in the superframe.
As a transmitter apparatus for a data transmission system using multicarrier modulation, an embodiment of the invention includes: a buffer that receives and stores a data quantity to be transmitted; a first insertion unit that determines a first quantity of dummy data and inserts the first quantity of dummy data into the data quantity supplied from the buffer to produce an enlarged data quantity; an error correction unit that receives the enlarged data quantity and performs redundancy coding to produce a redundancy data quantity; a data symbol encoder that receives the redundancy data quantity to be transmitted and encodes bits associated with the redundancy data quantity to frequency tones of a frame; a multicarrier modulation unit that modulates the encoded bits on the frequency tones of a frame to produce modulated signals; and a digital-to-analog converter that converts the modulated signals to analog signals. Optionally, the transmitter apparatus can further include a second insertion unit that determines a second quantity of dummy data and inserts the second quantity of dummy data into the redundancy data quantity supplied from the error correction unit to produce a modified redundancy data quantity.
The advantages of the invention are numerous. One advantage of the invention is that error correction (e.g., FEC) codewords are decoupled from frames (or symbols) as well as superframes. The result is that user data rate is made more independent from system performance and thus can be readily set in accordance with industry standards. Another advantage of the invention is that only minimal overhead is needed and that high coding gains can be maintained.
Other aspects and advantages of the invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.